The present invention relates to magnetic field controllers and in particular to a dc operated magnetic field controller of the magnet subsystem of mass spectrometers. The controller provides magnetic field measurement and field control of the mass spectrometer electromagnet.
Mass scanning on a mass spectrometer may be performed by varying the electromagnetic field of a large magnet. A magnetic field controller, with a hall effect sensor, controls the magnetic field and compensates for magnet hysteresis. The current technology used in the design of laboratory field regulated magnet systems is based on the alternating current (ac) design techniques. These ac based designs require complex circuitry to provide signal chopping, amplification, synchronous demodulation of the signal, and appropriate filter circuitry to remove the undesired ac components. This approach was required to provide stable, high gain, drift free hall sensor voltage amplification.
The mass analyzer of the mass spectrometer is built around a large electromagnet. Mass scanning is performed by varying the magnetic field. To vary the magnetic field the amount of current flowing through the magnet windings is changed. The magnet power supply is the source of the unregulated current. The current is conditioned and regulated by the magnet power supply regulator. Additional field regulation is required for precision analysis. This field regulation is provided by the magnetic field controller. The magnetic field controller is used in the feedback loop with the magnet power supply regulator to correct for magnetic hysteresis and other errors in the current regulation.
The function of the magnetic field controller can be described as follows. When the field control potentiometer setting is changed, the magnetic field controller senses an unbalance condition in the magnetic feedback loop and generates an error voltage, the power supply regulator modifies the current flowing through the magnet and the magnetic field is changed. The output of the hall probe senses the change in the magnetic field and compares it to the desired field as set with the field control potentiometer. The error voltage approaches zero as the actual magnetic field approaches the desired field and a balanced condition returns. The time required for the magnetic field to stabilize after adjusting the field control potentiometer is primarily dependent upon the magnet time constant--usually 2 to 3 seconds. Until recently, these required parameters were not easily obtained with integrated circuit operational amplifiers.